Motors which operate on a variable reluctance principle produce torque by switching current in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor so that the magnetic force of attraction between the rotor and the stator poles results as they approach each other. Each phase winding is wound around pairs of diametrically opposed stator poles. The current in each of the phase windings is switched off at an angular position before the rotor poles nearest the stator poles of that phase rotate past an aligned position which has a minimum reluctance between the aligned poles. Failure to switch the current at the aligned position results in a torque opposite to the direction of rotation of the rotor. Quick reduction of phase current is known to avoid negative torque. See U.S. Pat. No. 4,500,824.
FIG. 1 illustrates a block diagram of a prior art control circuit for controlling the switching of current flow in windings of a multiple phase variable reluctance motor. The control circuit controls the current flow from a DC voltage source 12 having positive and negative terminals 14 and 16 respectively through phase windings A-C. A switch controller 18 commutates switches Q1-Q4 to control the flow of current sequentially through phase windings A-C. The control of current through phase winding A is produced by generating switching signals Q1 and Q2 which forward bias transistors Q1 and Q2 to cause current flow from positive terminal 14 through transistor Q1 through phase winding A through transistor Q2 to negative terminal 16 and removing the switching signals Q1 and Q2 at the commutation point before rotor poles nearest the stator poles of that phase rotate past the aligned position between the poles of the rotor and stator where minimum reluctance exists in the magnetic circuit between the stator and rotor. Thereafter switching signals Q1 and Q3 are generated by the switch controller 18 to forward bias the transistors Q1 and Q3 to cause current flow through phase winding B and thereafter turn off switches Q1 and Q3 in the same manner as switches Q1 and Q2 described above. Finally, switching signals Q1 and Q4 are generated to forward bias transistor switches Q1 and Q4 to cause current flow through phase winding C in the same manner as phase windings A and B. This process repeats for each revolution of the rotor. As the rotor rotates each of the phase windings A-C are sequentially commutated by the switch controller 18 to establish current flow from the positive terminal 14 of the power supply 12 to the negative terminal 16 such that the current in each of the phase windings A-C is switched off in each phase at the commutation point before the rotor poles nearest the stator poles of that phase rotate past the aligned position representing minimum reluctance between the poles of the rotor and stator. Freewheeling diodes 20 are respectively connected between a first terminal 22 of each of the phase windings A-C and the positive terminal 14 of the power supply 12. A first diode 24 connects the negative terminal -6 of the power supply 12 to a second terminal 26 of each of the phase windings A-C. The function of the freewheeling diodes 20 and first diode 24 is to provide a return path for current which is flowing in each of the phase windings A-C at the time of turn off of the pair of switches controlling the flow in that phase winding. Initially when the pair of switches connected to each phase winding A-C is forward biased current flows from terminal 26 to terminal 22 to cause a voltage drop from terminal 26 to terminal 22. At the time of turning off the pair of switches which are connected to the phase winding in which current is flowing, a positive potential is induced at terminal 22 with respect to terminal 26 of the phase winding which forward biases the freewheel diode 20 connected to the phase winding and diode 24 to cause current flow from terminal 22 through diode 20 through the power supply 12 and through diode 24 to terminal 26. The induced positive potential at terminal 22 with respect to terminal 26, which opposes the reduction in current caused by initiating turning off the switches slows down the stopping of current flow in the phase winding connected to the switches which are being turned off. Slowing down the turn off of current flow in a phase winding prevents the generation of maximum torque by the phase winding as a consequence of the requirement that the turn off point of the switches connected to the phase winding must be advanced with respect to the minimum reluctance position of the poles of the rotor and the phase of the stator being turned off. Retarding the turning off point of the switches controlling current flow in a phase winding as close as possible to the minimum reluctance position increases the torque generated by that phase winding. Accordingly, speeding up the turning off of current flow in the phase winding under the control of the switch controller 18 increases the torque generated by the motor.